Road Salt Sensor for Better Highway Safety, Less Environmental Damage

Common salt glows orange under ultraviolet light, showing road-salt crews where enough is enough

2 min read
Road Salt Sensor for Better Highway Safety, Less Environmental Damage
Spreading more salt on roadways that already have enough for safety wastes salt and money, and increases damage to the infrastructure and environment.
Photo: Tom Gralish/The Philadelphia Inquirer/AP Photo

Spreading salt on roadways is still the most cost-effective tool for preventing death, injury, and property damage from winter traffic accidents around the world.

In 2011, highway departments in the U.S. alone spread 19.6 million tons (1.8 billion kilograms) of salt on icy roads. They pay anywhere from $37 to $130 a ton for rock salt, so the total price tag is in the $1 billion to $2 billion range. But that’s just the beginning of the cost. Overall, for every dollar spent on the salt itself, there may be three dollars in damage—to roads, vehicles, and the environment, as salt eats away at bridge decks, corrodes rebar, blights neighboring vegetation, and drains off into surface and groundwater.

Spreaders lay down salt repeatedly during the icy season. Sometimes they may throw down more rock salt on areas that already carry far more than the optimum 15 grams per square meter; at other times, they may not drop enough. The Salt Institute says that roads departments could cut groundwater salt levels in half through careful controls.

Accurate, real-time measurement of the amount of residual salt on the pavement is vital to maintaining safety and reducing both direct costs and collateral mayhem. There are already sensors that collect water splashed back by a salt truck’s wheels and measure the changes in refractive index that accompany increasing salinity. These systems, first described a decade ago, work well…if the road is wet.

In the current issue of Sensors and Actuators B: Chemical, engineers at Spain’s Universidad Carlos III de Madrid demonstrate a simple, robust optical sensor that can strap onto a bumper and measure dry-pavement salt levels as the truck drives over dry pavement.

Salt glows orange in black light, and the brightness of the glow is proportional to the amount of salt present. More precisely, when illuminated at 273 nanometers, sodium chloride fluoresces at 610 nm (and also, though somewhat less usefully, at 310 nm).

The instrument the researchers describe includes an ultraviolet LED (much lower-powered than the lasers required by some other methods) and a photoreceptor, some filters for both emitter and receiver to tighten the ranges of stimulus and response, a power supply, A/D and D/A converters, a field programmable gate array (FPGA) to control the components and analyze the results, and a power supply that can modulate the LED output—all in a compact, rugged three-board package.

The researchers focused on salt’s 610 nm emission band: the wavelength is far enough away from the excitation wavelength to reduce backscatter effects, and this wavelength avoids the fluorescence of some of the biological material that collects on roads.

Engineers Marta Ruiz-Llata, Pedro Martín-Mateos, José R. López, and Pablo Acedo used a modulated UV LED to help them discriminate between reflections (from the UV and environmental sources) and the 610 nm fluorescence. It’s a balancing act: the slower the emitter fluctuation, the brighter the fluorescence, but the lower the ability to sort the fluorescence signal from reflections. (Fluorescence does decline very, very slightly with increasing temperature—but the change was small compared to other sources of variation, especially the naturally non-uniform distribution of salt broadcast over a hard surface.)

They adopted a 10 Hz excitation cycle (at this rate, the fluorescence persists for about 14 milliseconds). Tests showed that the device accurately measures the concentration of salt to within 10 percent over the critical range from 0 to 20 grams per square meter.

Photo: Tom Gralish/The Philadelphia Inquirer/AP Photo

Edited 5 February 2014: Instituting controls and best practices helps highway departments cut salt in groundwater in half, rather than halving overall salt usage. 

The Conversation (0)

We Need More Than Just Electric Vehicles

To decarbonize road transport we need to complement EVs with bikes, rail, city planning, and alternative energy

11 min read
A worker works on the frame of a car on an assembly line.

China has more EVs than any other country—but it also gets most of its electricity from coal.

VCG/Getty Images
Green

EVs have finally come of age. The total cost of purchasing and driving one—the cost of ownership—has fallen nearly to parity with a typical gasoline-fueled car. Scientists and engineers have extended the range of EVs by cramming ever more energy into their batteries, and vehicle-charging networks have expanded in many countries. In the United States, for example, there are more than 49,000 public charging stations, and it is now possible to drive an EV from New York to California using public charging networks.

With all this, consumers and policymakers alike are hopeful that society will soon greatly reduce its carbon emissions by replacing today’s cars with electric vehicles. Indeed, adopting electric vehicles will go a long way in helping to improve environmental outcomes. But EVs come with important weaknesses, and so people shouldn’t count on them alone to do the job, even for the transportation sector.

Keep Reading ↓Show less